Kraft-simulating pulp and process of producing same



Patented Feb. 17, 193i r, OFFICE GEORGE a nrcrrrnn, or BERLIN, NEW HAMPSHIRE, assumes no Brown COMPANY, or BERLIN, mew HAMPSHIRE, A conronarron' or MAINE KRAFT-SIMULATING PULP AND PROCESS 9F PRODUCING SAME {No Drawing. Application filed July 1,

This invention has relation to a novel pulp product possessing physical characteristics and a chemical composition substantially similar to kraft or sulphate pulp but which,

1 however, is produced according to the pres-' ent invention by a process quite dissimilar to the kraft or sulphate process,-so-called. For convenience of designation, I will hereinafter term the pulp produced in accordance with the present invention a kraitsimulating pulp in contradistinction to pulps produced by the -kraft or sulphate process.

In order to give a better understanding 16 of the present process and the kraft-simulating pulp product secured thereby, it is necessary to consider, in some detail, the subject matter of my application for United States Letters Patent, Serial No. 749,564,

filed November 12, 1924. In that application I havedisclosed a process for producing pulp characterized by its extraordinary high Mullen or bursting strength and light color. The salient feature of discovery outlined therein, generally stated, is that if Wood chips. (spruce, for example) are cooked in an acid sulphite solution in which the free S0 and the combined SO are in substantiall equal proportions by weight (or the free 0 less in proportion than the combined S0 but are present in suflicient quantity in solution to act upon the encrusting or cementitious content of the wood for a sufiicient period of time in a closed digester'at atemperature higher than that ordinarily employed in sulphite digestion, a pulp of extraordinary strength and light color is produced. In practising the process, I employ a sodium bisulphite solution (or an equlvalent bisulphite solution of zinc, magnesium, potassium or other suitable metal) in which the combined S0 and free S0 are in substantially equal proportion, say 3% to 4% of each. Ihe

digestion of the chips is carried out in a closed digester at a temperature of 320 F.

and a pressure of 75 to 95 pounds for about ten to eleven hours. After the completion of the digestion, the digester contents are blpwn into a suitable blow pit, the pulp is separated from the spent liquor and 15 1925. Serial No. 40,958.

washed, and, if desired, screened as or- (hnar1ly. v

The pulp which results in the practice of a 4-4 digesting process, that is, a process content, however, the pulp is diflicult to bleach, requiring 30% to of 35% bleach,

, based on the bone-dry weight-of fiber, and

such bleaching operation resulting in a product which has a materially lower pentosan and lignin content and accordingly a lower strength (one which is comparable to that of ordinary bleached sulphite pulp), so that ordinarily it wouldnot be economical or ex pedient to produce a bleached product therefrom in competition with the usual sulphite pulp for the manufacture of high grade writing papers. Moreover, although the unbleached 4-4 pulp, as such, would be particularly applicable for producing heavy papers for use in the manufacture of bags,

wrapping paper, etc, it is ordinarily unsalable for such because of its deficiency in tear test and because the trade generally associates heavy and strong papers with a kraft or brown color and are prejudiced as to the value and strength of a light-colored product for such usuage.

I have discovered that if the light-colored pulp produced by a 4-4 acid sulphite digestion of raw cellulosic material is subjected to treatment at an elevated temperature in an alkaline solution of the proper strength for a sufficient period of time, the pulp undergoes a color change or coloration, assumin g a brownish color similar to kraft or sulphate pulp. In such treatment, the alkaline solution must be of sufiicient strength to cause the coloration or color change substantially without affecting or changing the fiber strength of the pul or, in other words, without removing or re ucing materially the pentosan contest thereof, since otherwise its most important kraft-like property of strength will have been destroyed. Moreover, in addition to causing a coloration of the pulp I have found that the alkali treat- I ment tends to remove less-resistant beta and gamma celluloses and'also to impart a materially increased softness to the pulp which, when tested, accordingly, shows an increased tear test. Y

While I am unable to account precisely as to the theory or mechanism of the chemical reaction causing the coloration, I have reason to believe that the alkaline agent probably reacts with the lignin and pentosan in the pulp to produce a kraft-brownish color or reaction product. Such a conclusion appears logical in view of the analogous colored pulp products formed in alkaline digestion, as for example in the kraft or-sulphate process and in the soda process. Whatever the explanation or theory involved in such coloration be, b the practice of my invention I am enable to produce a pulp which is subtantially similar to kraft in both chemical and physicalbehavior but by a combination acid-alkali treatment radically dissimilar to the kraft process and having certain very important advantages which will subsequently be ointed out.

n the manufacture of the pulp of the present invention, the wood chips to be digested, spruce for example, are placed in a digester in an acid sulphite liquor having a relatively high combined SO content, say 3% to 4%, and cooked under the requisite temperature, pressure, and time conditions hereinbefore indicated and more fully set forth in the copending application for the production of a pulp product characterized especially by its unusually high strength and pentosan content. In certain cases, however, it may be desirable to commence the digestion of the chips raw cellulosic material.

in an acid liquor containing say, about 7% free S0 and 4% combined $0 and relieve the digester of the excess free S0 during the early stages of the cook. Such procedure, while substantially equivalent to that outlined, gives better penetration of the SO, into the chi s and tends to promote a more rapid and e cient liberation of fiber. I desire to make it clearly understood that where a kraft-simulating pulp is the ultimate product aimed at, not only low-resinous woods such as birch, beech, maple and hemlock, but also resinous woods frequently employed in. the manufacture of kraft pulp, such as j ackpine, long leaf pine, .etc., may serve as the Any resinous or gummy material in a resinous wood which might survive the 4-4 acidsulphite digestion and which might ordinarily interfere with paper-making operations on the Fourdrinier wire or felt of the paper-making machine is removed by the subsequent alkaline digestion. A

Due to the higher combined SO content than is ordinarily present in the cooking of sulphite pul the hydrogen ion concentration or activity of free $0 is suppressed sufficiently to avoid the usual rather drastic hydrolysis which occurs in the cooking of sulphite pulp. This results in a pulp of greater average fiber length and a higher pentosan and hgneous content than ordinarily in sulphite digestion; that .is the pulp contains more of the fiber and is produced with a higher yield than if digested in an acid sulphlte liquor, wherein hydrolysis of the cellulose fiber occurs to a greater extent, or even in an alkaline liquor.

The foregoin analysis of what takes place is substantiate by actual data. For example, in ordinary commercial operation with the usual acid bisulphite di esting liquor the yield is from 43% to 47% of the weight of the dry wood. In ordinary sulphate or soda digestion the yield is from 40% to 47% of the wei ht of the dry wood, whereas in the 4-4 aci sulphite treatment, on the other hand, the yield is from 50% to thus permitting a subsequent alkaline digestion with the final kraft-simulating pulp yield, bein equal to about 85% of the 4-4 pulp yiel and the over-all yield from the wood chips being about equal to that obtained by the usual kraft process.

After the completion of the 4-4 acid digestion, the pulp is separated from the spent liquor; is washed as by passage through a counter-current washer of the type described in U. S. Letters Patent No. 1,421,664 to Brown et al., dated July 4, 1922; and is preferably screened, as ordmarily, by passage through the usual screens employed for pulp to remove shlves, specks and other like foreign material. The washed pulp is then thickened, as on a wet machine, and is intimately admixed with an alkaline solution, preferabl caustic soda (NaOH) or sodium sulphide Na s), to the consistency desired for carryin out the alkaline treatment. The pulp is igested in the solution at an elevated temperatureand is agitated or stirred during digestion so as to promote a uniform reaction for a sufficient period of time to allow the brown coloration and a softening of the light-colored pulp to be completed. I have found it more economical and expedient to carry out the alkaline digestion in open tanks under atmospherie pressure and to maintain an elevated temperature by blowing live steam into the tanks, but I desire to have it understood that the alkaline digestion may be carried out in closed (ligesters at high pressure and temperature and yield equally satisfactory results. The alkaline digestion does not materially affect the strength or pentosan content of the pulp, but merely causes a coloration thereof and imparts an increased tear test thereto. In the alkaline digestion of a .4-4 pulp produced from resinous Woods, the resinous and gummy material which may have survived the 4-4 digestion is to a great extent removed so that trouble in subsequent paper-making operations is not encountered. After the brown coloration of the pulp has taken place to completion, the pulp is washed, as by passage through a counter-current washing machine of the type hereinbefore indicated. The spent liquor presents a yellowish or brownish appearance and perchance a darker appear ance in the case of highly resinous woods.

The washed pulp presents an appearance not unlike that of ordinary or commercial kraft pulp.

The following example will sufiice to give a more comprehensive description of a preferred method of procedure falling within the purview of the present invention so as to allow those skilled in the art to practice the same. In carrying out the alkaline digestion, the washed and preferabl screened 4-4 stock from the acid sulphite i estion of wood chips .is'idigested at a stock ensity of about 10% in a caustic soda solution containing at least about 3% to 4% and not more than about 10% to 12% caustic soda based on the dry weight of pulp.- The pulp is digested inopen tanks under atmospheric pressure at a temperature of about 180 to 212 F., preferably while being agitated, from two to seven hours, or until the desired reaction is produced in the pulp, the time required therefor depending upon the characteristics of the particular stock undergoing treatment and the temperature employed in digestion. A coloration and increase in tear test is produced by such alkaline digestion substantially without afi'ectin the strength of the pulp. The digested pu p is finally washed free from al kali and may be employed in the production of high-grade heavy papers suitable for the manufacture of bags, wrapping paper and like products which not only approximate the physical and chemical composition of krai't products but have the color thereof.v

It is again to be emphasized that in the combination treatment of the present invention, the yield of kraft-simulating pulp obtained is at least equal to the yield of kraft pulp resulting from the digestion of wood chips in a kraft or sulphate liquor. A great advantage in the manufacture of my kraft-simulating pulp is that it can be made in any locality because of the use of unobjectionable methods of manufacture. As is known, kraft pulp can only be made in sparsely settled communities removed from large cities due to the undesirable mercaptan odor arisin from the recovery of chemicals or in the 'gesting process. My inven- Kraft-simulilting pulp Usual kraft 4-4" or 4-4' pulp or sulpulp after phate" alkaline pulp treatment Strength (beaten) 160-165 154-l00 155-160. slowness (beaten) 4.59-6 min. 11.2-13 min 7-10 min. Al ha cellulose 84.38- 89.41- 89.90. Fl 1' length (cm) .96 79-.85 .7-.90. 'Ic 144-150 257-290 220-245. Pentosancontent 6.0-7 5.6l6.5 7-10. Pulp yleld (based on initial wood) 54-55% 5-46 42-45%. Color Light Kralt-like Kraln From the foregoing table, which represents the average of a number of tests, it will be seen that the kraft-simulating pulp yield is at least equal to that resulting from the kraft or sulphate rocess of digestion and that the strength, 5 owness, alpha cellulose content, fiber length, and color thereof are about equal to that of kraft. It is to be noted that the alkaline treatment only lowers the pentosan content of the which is a trifle lower than that of kraft pulp, and raises the alpha cellulose content thereof to that of kraft pulp, probably because of the removal ofligneous matter, less resistant celluloses and resinous material therefrom.

Before concluding, it=may be stated that the results realized in the ractice of the present invention cannot be obtained by an alkaline digestion of wood chips followed b a 4-4 treatment. The essential feature lies in the alkaline treatment subsequent to the 4-4 digestion.

I What I claim is:

1. A process of producing a kraft-simulating pulp, which comprises digesting wood chips at elevated-temperature and pressure in an acid sulphite liquor in which the free S0 and combined S0 are in approximately equal proportions of 3% to 4% each, separating the resulting pulp from its digesting liquor, and digesting such predigested pulp in an alkaline cooking liquor. V

2. A process of producing akraft-simulating pulp, which comprises digesting wood chips at elevated temperature and pressure in an acid sulphite liquor in which the free S0 and combined SO are in approximately equal proportions of 3% to 4% each, separatslightly -4 pulp,

ing the resulting pul from its di esting 1iquor, and cooking suc predigeste pulp in a caustic soda solution containin from about 3% to 12% caustic soda, base on the dry 5 weight of pulp.

In testimony whereof I have afiixed my signature.

GEORGE A. RICHTER; 

